Upcoming neurostimulation applications (e.g., implantable neurostimulators) are demanding pulse generator architectures that can deal with multiple electrodes which are active at the same time, inject large currents and support high pulsing rates, with reduced electrode areas (to improve selectivity) and without interruption of stimulation. These constraints require systems and methods that can utilize information about the charge injection process to transparently modify stimulation patterns in order to avoid excessive electrode potential excursions and deal with mismatches between stimulating and return currents (used for active charge balancing) in order to avoid runaway issues in the DC blocking capacitors in series with each electrode. Managing the charge injection process can also permit minimizing the stimulus artifact (SA), allowing robust sensing of evoked responses caused by electrical stimulation, and possibly enabling closed-loop neurostimulation applications.
In traditionally programmed charge-balanced stimulation, the charge injected by a pulse generator in a stimulation pulse is followed by a balancing pulse of equal and opposite charge. This methodology assumes only reversible chemical reactions occur during therapy delivery, which may not be the case. For unipolar cathodic stimulation, E. Hudak proposed in “Electrochemical Evaluation of Platinum and Diamond Electrodes for Neural Stimulation” (PhD Thesis, Dept. of Chemical Engineering, Case Western University, May 2011) that a large value resistor may be placed across the DC blocking capacitor in series with the stimulating electrode to bleed off a portion of the stimulation charge, forcing the balancing anodic phase to have less charge, in an attempt to compensate for irreversible chemical reactions that may occur during therapy.
However, DC blocking capacitors are an important safety feature in the design of pulse generators. They are primarily used to limit the charge per phase, to reduce DC leakage, and to prevent DC current from circulating through tissue under a fault condition, among other functions. Although various approaches have been proposed for their elimination (as their size is an impediment to implant minimization), they continue to be employed in pulse generators owing to the safety they provide. Thus, placing a resistor across a DC blocking capacitor as proposed by Hudak is in contrast to the main purposes of the blocking capacitor, and may therefore impact compliance with tight DC-leakage requirements (e.g., 100 nA maximum) of clinical implantable active devices. Moreover, the compensation proposed by Hudak is open loop, causing the shift in electrode potential to be either positive or negative with respect to the open circuit potential (OCP) depending on the amount of imbalance generated by the charge bleeding.
Further, U.S. Pat. No. 6,301,505 B1 describes an electrical tissue stimulating device which includes circuitry for monitoring the build-up of undesirable residual voltages between stimulating electrodes and reducing such voltages in the event the condition occurs.
US 2011/0125217 A1 also describes measuring any residual charge remaining in an electrode that may result from an imbalance in the applied stimulation. More particularly, US 2011/0125217 A1 describes measuring the voltage across a DC blocking capacitor in series with an electrode to provide an accurate representation of the integral of the charge flow to the electrode, and thereby provide a measure of the residual charge on the electrode contact.
US2008/0015641 A1 describes electrical stimulation via electrodes, wherein the presence of a residual charge at the electrodes is measured via a differential voltage measurement, and action is taken in case thresholds are crossed.